A secondary battery is an energy storage that is basically made up of positive electrode/negative electrode/separator/electrolyte solution and can be recharged by reversible conversion between chemical energy and electrical energy, ensuring high energy density. The application of secondary batteries is rapidly expanding to small electronic devices including mobile phones and laptop computers, and even hybrid electric vehicles (HEV) and energy storage system (ESS).
A secondary battery is an electrochemical device that is insulated by a separator and thus is stable, but there is a risk of heat generation and explosion in the even that a short circuit occurs between a positive electrode and a negative electrode due to internal or external battery failure or impacts, so the most important consideration is to ensure thermal/chemical safety of the separator as an insulator.
However, a separator made of polymer including polyolefin has high thermal shrinkage at high temperature and a risk of failure caused by dendrite growth. To solve the problem, disclosed is a coated separator in which one surface or two surfaces of a porous separator substrate are coated with inorganic particles along with a binder to protect the separator from a risk of failure and prevent thermal contraction.
Regarding the coated separator, according to Korean Patent No. 10-0775310, a polymer resin binder and inorganic particles are added to an organic solvent to prepare an organic/inorganic slurry (PVDF-CTFE/BaTiO3 or PVDF-CTFE/Al2O3) and the organic/inorganic slurry is coated on a porous substrate to manufacture a separator having an organic/inorganic composite porous layer. In this process, a binder solution containing binder resin such as PVDF-CTFE dissolved in a solvent is used to provide good adhesion between powdery inorganic particles. In this case, however, because the binder solution is apt to penetrate into the pores of the porous substrate, a large amount of binders need to be used to show sufficient adhesion between the inorganic material and the porous substrate surface, resulting in battery performance degradation. FIG. 2 shows a separator having a composite porous layer according to prior art, and as in the illustrated example, the binder resin dissolved in the organic solvent penetrates into the pores of the porous substrate and blocks the pores of the porous substrate. Moreover, as the binder concentration in the slurry increases, the slurry viscosity greatly increases, making it difficult to form a thin-film organic/inorganic composite layer and high temperature is required in the drying process, and when the slurry viscosity is maintained at low level, adhesion with the porous substrate or adhesion between the inorganic material is lowered, causing the inorganic particles to easily detach.
As the organic solvent-based process involves a long dry zone of a dry line during drying due to the critical explosion limit, the slurry changes in concentration and rheological properties by solvent evaporation in the slurry preparation and transfer and coating processes, which may affect the coating quality of final products.
Furthermore, to increase adhesion between the porous substrate and the organic/inorganic composite porous layer, Korean Patent No. 10-1125013 discloses a method for manufacturing a cross-linked ceramic-coated separator using ionic polymer that dissolves in water. This method also uses ionic polymer that is soluble in water, but not disperse and completely dissolves it in water, inevitably encountering a phenomenon in which the solvent is trapped, and a larger amount of organic solvent such as diethylacetamide solvent is used 15 times than water, failing to provide a technical motivation to a coating method using an aqueous solvent, and besides, it is necessary to add crosslinker and initiator together to an organic solvent in the slurry preparation process to induce chemical crosslinking after coating for the benefit of increased adhesion with the substrate, and heat or UV treatment for 20 hours or longer is absolutely required in the drying process. However, when crosslinker and initiator are added to the slurry solution, partial self-crosslinking takes place by heat and energy applied from the outside environment in the process of storage and transfer of the coating solution before applied to the porous substrate, promoting solidification of the slurry, eventually resulting in reduced uniformity of the coated separator. Furthermore, because long-time heat treatment and UV treatment is necessary during drying, the output of the manufacturing process is limited, and the thin-film porous substrate may be damaged by high temperature/high energy during drying, causing property degradation and air permeability reduction.